Well all spiders that bite you can be lethal, but it depends what you mean about poisonous. Does the poison go slowly through your veins are does it kill you instantly? So really all spiders are deadly but not many people get bitten by a spider its more wasps and bees.

Almost all spiders are venomous, with very few exceptions. However, of more than 50,000 known species worldwide a handful are significantly venomous to humans.

The standard measure of toxicology is the LD50 test (basically a lethal dose which kills more than 50% of your test animals, usually mice or rats) the two species of spider are top of the league; one of the so called Brazilian wanderers, Phonutria fera, and the Sydney Funnel Web, Atrax robustus. That said death is the most extreme reaction to the venom of these spiders and the amount of venom injected varies. The recorded human deaths for these species over the last 50 years appears to about 15-20 each.

To put this in perspective - without doubt there are a greater number of deaths attributed each year to any colour car you care to name. Golf balls are probably also more dangerous.

That's a valid point Drosophilla - I do not think that where the LD50 test is concerned that the potential for variable effects between species is taken into account in any meaningful way. That said I guess that if a toxin kills more than 50% of your mice this is taken as a reasonably good indication that it will have a fairly nasty effect on us too. However, in the case of the Sydney Funnel Web my understanding is that this venom has a negligible effect on dogs. Perhaps there is argument for toxicity test on humans, all in the cause of science of course - any volunteers?